Deceleration apparatus for ribbon and spot beams

ABSTRACT

A deceleration apparatus capable of decelerating a short spot beam or a tall ribbon beam is disclosed. In either case, effects tending to degrade the shape of the beam profile are controlled. Caps to shield the ion beam from external potentials are provided. Electrodes whose position and potentials are adjustable are provided, on opposite sides of the beam, to ensure that the shape of the decelerating and deflecting electric fields does not significantly deviate from the optimum shape, even in the presence of the significant space-charge of high current low-energy beams of heavy ions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/280,162 filed Oct. 24, 2011, which claims the benefit of U.S.Provisional Application No. 61/405,878, filed on Oct. 22, 2010 andentitled DECELERATION CHAMBER FOR RIBBON AND SPOT BEAMS, both of whichare hereby incorporated by reference in their entirety for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is concerned generally with a deceleration apparatuscapable of filtering out neutral particles, and more particularly, adeceleration apparatus capable of producing either a short spot beam ora tall ribbon beam with good beam angle control and smooth profiles.

2. Description of Related Art

Ion implantation is a process used to introduce into a target substrateatoms or molecules, generally referred to as dopants, to make materialswith useful properties. Of particular interest, ion implantation is acommon process used in making modern integrated circuits. Recently,interest has focused on generating ribbon beams of over 300 mm in sizecontaining milliampere currents of ions at energies as low as 200 eV.

The highest beam currents are obtained by decelerating the ion beamimmediately prior to the target; however, this practice has severalknown disadvantages. One disadvantage is that the deceleration tends tomodify the trajectories, magnifying any angular errors and makingcontrol of uniformity in a ribbon beam more difficult. Anotherdisadvantage is that a portion of the ions is neutralized bycharge-exchange processes with residual gas atoms and molecules and, asa result, is not decelerated. These ions penetrate into the silicon muchfurther than is intended, and this deep penetration of some of thedopant ions interferes with the intended process; furthermore, sinceneutralization depends on system pressure within the vacuum system, itis difficult to maintain constant conditions from day to day, and thelevel of contamination is not sufficiently constant to be tolerated.

Many recent innovations to ribbon-beam implanting systems are discussedor disclosed in U.S. Pat. No. 7,902,527, which is incorporated herein byreference. Key content of this patent is summarized below.

Some implanters use a lens to halt the divergence of the ion beam onreaching the requisite major dimensional size, and to collimate it, i.e.render it parallel. A suitable lens may use magnetic or electric fields,may generate a quadrupole field, and must have a beam passage of highaspect ratio to conform generally to the ribbon shape of the ion beam.

In certain circumstances such as when using high-current low energybeams it may not be possible to reliably deliver a ribbon beam that issufficiently uniform, so the '527 patent discloses a two-modeimplantation system. This comprises two multipole lenses after theanalyzing magnet. In a first mode, the currents in the coils of onemultipole lens can be controlled responsive to a measurement of the ionbeam profile to control the current density in this beam profile. Theion beam is allowed to continue as a ribbon-shaped beam whose majordimension exceeds a dimension of the workpiece. The workpiece is thentranslated through this ion beam along a single path, one or more times,to implant a desired uniform dose of ions into its surface. In a secondmode, the currents in the coils of a first multipole lens are excited soas to generate a quadrupole magnetic field which causes the ribbon ionbeam to converge in its major dimension, thereby generating at adownstream location a beam spot which is smaller in both dimensions thaneither dimension of the workpiece—referred to hereinafter as a ‘spotbeam’. The workpiece is then translated in a reciprocating path in twodimensions through the ion beam, so as to implant a uniform dose of ionsinto its surface by implanting a succession of partially overlappingstripes. This is referred to as two-dimensional scanning.

It is generally desirable to minimize the number of passes required inorder to achieve a specified dose uniformity. Commonly a standarddeviation of 1% or less of the overall dose is an acceptable uniformity.The uniformity achievable depends upon the shape of the ion beam,specifically in its projection in the direction of the striping, or the‘slow scan direction’. The profile of the beam needs to be a smooth‘bell curve’. If it contains spikes or valleys in the profile, thesewill cause an increase in non-uniformity, which can be offset by a largeincrease in the number of stripes. However, increasing the number ofbeam passes will decrease the throughput, so is less economicallyviable.

The second mode is likely to be advantageous when using high-current,low-energy beams (for example greater than 1 mA at energies below 3keV), under which conditions space-charge and other effects makepositive control of the uniformity of the current in a beam moredifficult. The first mode requires slower motions and is likely todeliver higher processing throughput at energies where satisfactorycontrol of the ion beam profile can be achieved. The currents in themultipole lens in either mode may be adjusted to fine-tune the beamcurrent density profile of the beam, even though at low energy thiscontrol is insufficient to ensure a uniform implant in one pass with aribbon beam. In the second mode, this may be valuable to generate abell-curve profile.

The '527 patent further discloses a second lens after the firstmultipole lens, whose function is to collimate the ion beam. This isparticularly important for the first operating mode, i.e. theribbon-beam case, where systematic variation in the implant angle acrossthe face of the workpiece would otherwise occur. It is also of value toreduce the range of angular variation in the ion beam in the secondmode.

The '527 patent further discloses optional means of deceleration oracceleration of the ion beam using a bent ion beam path, to deliver highbeam currents at low energies while filtering out contaminants with thewrong energy, for use in ion implantation in either the ribbon-beam or2D scan beam modes. The beam is bent through an angle that differs by asmall amount from standard conditions, then the ion beam is deceleratedby means of a set of electrodes that superimpose two opposed successivesideways components of electric field on the deceleration field, so thatthe ion beam is deflected in an s-shaped bend, the deflections eachamounting to an angle of at least 10 degrees, and providing a lateraldisplacement of several times the width of the ion beam, returning it toits original path. By providing beam stops on either side of the beam,the only ions transmitted are those with the correct charge and energy,so contaminants with the wrong energy or charge can be removed. Suchcontaminants include neutral atoms formed from beam ions by chargeexchange with the residual gas, and since the cross-sections for somecharge-exchange reactions peak at beam energies below 1 keV, thisbecomes very important. This deceleration means has been described as a‘chicane’ deceleration scheme.

BACKGROUND TO THE INVENTION

When decelerating a ribbon beam, it is very important that no componentof electric field appear in the direction of the major transverse axisof the ribbon beam. Existing systems use electrodes with planarsymmetry, i.e. a shape which could be drawn on a plane in which thes-shaped trajectory of an ion should lie, and which is then extrudedalong the direction of the major dimension of the cross section of theion beam. Unfortunately it has been found that such transverse fieldcomponents can still appear, for a variety of reasons which include:

a) The ribbon-shaped beam may be limited in size by passing it through arectangular aperture. If this aperture is too close to the strong fieldsin the deceleration system, it modifies the shape of the fields near thebeam extremities, causing unwanted beam deflection.

b) The electrode shapes are conceived as extrusions, but they are finitein extent, and the fields will be perturbed at their ends. Typically theelectrodes must stop at a point within a metallic vacuum chamber at adefined electrical potential, such as ground potential. This createsstrong local electric fields which may penetrate some distance insidethe region the beam may occupy, disturbing the electric field at thebeam edges.

c) The effects of the beam space-charge modify the potentialdistribution, and it would be desirable to confine these effects to thegeneration of fields in the direction of the minor beam dimension, tocause them to vary linearly with position across the beam profile, or toeliminate them altogether. Otherwise these fields tend to cause verynon-linear defocusing at the edges of the ion beam, leading to alowering of ribbon beam uniformity, and a loss of otherwise usable beamcurrent. For a spot beam, the effects will be stronger and thereforemore harmful. It would be desirable to maintain or enhance thesmoothness of the projection of the beam current density so as todeliver a beam spot with a ‘bell-curve’ profile in the same direction asthe major dimension of the ribbon beam.

The present invention is an improvement to earlier equipment and methodsused to decelerate ions through a ‘chicane’ deceleration system, orindeed other similar deceleration or acceleration systems, mitigatingthe effects which can adversely affect the transport of an ion beam,either spot-shaped or ribbon-shaped, through these systems. It addressesthe need to provide a uniform, high current ribbon beam, oralternatively, a spot beam with a smoothly varying current-densityprofile.

SUMMARY OF THE INVENTION

The present invention involves an assembly of prism-shaped electrodesbetween which an ion beam is passed, undergoing deflection anddeceleration as a result of potentials applied to the electrodes. Anumber of factors can distort the shape of the equipotentials and theelectric fields.

One aspect of the present invention is to provide conducting electrodecaps mounted to and connected to at least some of the prism-shapedelectrodes, which substantially close the gap at the ends of theelectrodes, leaving only a sufficient gap safely to withstand thepotential difference between the electrodes, but substantially shieldingthe beam from the chamber potential.

Another aspect of the present invention consists of widening the gapsbetween the electrode caps described above, and providing a further pairof electrodes, one each in the gap at the ends of the prism-shapedelectrodes and between the caps, and applying an intermediate potential,thereby greatly reducing the magnitude and extent of non-uniformity ofthe electric field caused by the cap electrodes themselves.

Another aspect of the present invention is to control the intermediatepotential applied to said further pair of electrodes, responsive to thelevel of space-charge in the beam, so as to mitigate the tendency ofspace-charge blowup to occur along the direction of the axis of theprism-shaped electrodes, and to provide some control of the currentdensity of the beam. The current density can be measured by well-knowntechniques such as a traveling Faraday cup.

Another aspect of the present invention is to provide a means of movingsaid further pair of electrodes closer to or further from the ion beampassing between them, to allow control of space-charge blowup and of thebeam size, applicable to ion beams of different sizes.

Another aspect of the invention is an improvement to a decelerationapparatus for deflecting an ion beam and decelerating it from a firstenergy to a second energy. The apparatus will not decelerate neutralatoms, nor will it deflect them, and it will deflect ions with anincorrect energy through a significantly different angle, thus allowingunwanted ions and neutral atoms to be intercepted and removed from thebeam. The faces of the electrodes which are close to the ion beam are ofprismatic form, extended in the direction which is orthogonal to thebeam travel direction, and also to the direction in which the beam isdeflected. The ions may be deflected twice, in opposite directions.

The improvement consists of providing additional symmetrical shapedelectrodes, in locations beyond the limits of the ion beam and onopposite sides of the beam mid-plane in which ions are deflected.Controlled potentials are applied to these electrodes. The electrodesare movable in the direction directed toward the beam axis, and may bemoved to selected positions a desired distance from the edges of thebeam. These electrodes affect the shape of the electric fields, and byadjusting the potentials and positions on these electrodes, they may beeffective to mitigate any disturbance to the shape of the electricfields due to the presence of grounded chamber walls or other conductiveitems near the deceleration system, or from space-charge forces causedby the beam itself. Thereby the uniformity of a ribbon beam beingdecelerated by be preserved or improved. Alternatively, the smoothnessof the current-density profile of a spot beam may be preserved orimproved.

Another aspect of the invention is an improvement to an ion implantercomprising a source of accelerated ions, a magnetic mass analyzer, oneor more multipole magnetic lenses, an ion beam deceleration apparatuslocated downstream from said mass analyzer, and a means of mechanicallyscanning a workpiece through the resulting ion beam, either in a singledirection or in a 2-dimensional raster pattern. The multipole lenses maybe used to form either a ribbon-shaped beam of substantial uniformcurrent density along its major dimension, which is orthogonal to saidsingle direction in which the workpiece is scanned, or a smaller spotbeam with a bell-shaped profile in the same direction. The decelerationapparatus comprises prism-shaped electrodes at different controlledpotentials, which create transverse electric fields to deflect the beamalong an s-shaped path while decelerating it, and causing the issuingdesired beam to be centered on an intended axis, along which it travelsa short distance before impinging on the scanned workpiece. The currentdensity profile, and optionally the angular distribution of the ions,are measured by means of a traveling Faraday cup, or alternatively by anarray of Faraday cups, or by equivalent means.

The improvement comprises providing at least one pair of movableelectrodes symmetrically disposed about the beam, to which acontrollable potential is applied, and of controlling the position andthe potential of the electrodes to enhance the beam shape: in the caseof a ribbon beam, to minimize the non-uniformity, and in the case of aspot beam, to enhance the conformance to a smooth bell-shape. In mostinstances the improvement is obtained by minimizing the generation ofelectric field components aligned with the major transverse dimension ofthe ion beam.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross section through the midplane of a prior artimplanter system utilizing a chicane deceleration system, to which thepresent invention may be applied.

FIG. 2a is a section through the midplane of a prior art chicanedeceleration system.

FIG. 2b is a side view of a prior art chicane deceleration systemshowing ion trajectories of a ribbon beam, and showing equipotentiallines.

FIG. 3 shows a partially cutaway perspective view of a chicanedeceleration system incorporating improvements according to the presentinvention.

FIG. 4 shows a section through a low-current ribbon ion beam passingbetween electrodes between which a deflecting potential is applied, withthe beam perturbed by the proximity to the ends of the electrodes and toa grounded chamber wall.

FIG. 5 shows a similar arrangement to FIG. 4, but caps are added to theelectrode ends to shield the beam from the external wall potential.

FIG. 6 shows a similar arrangement to FIG. 5, but with an additionalpair of electrodes at a median potential placed between the caps, toimprove the uniformity of the deflecting field near the beam edges.

FIG. 7 shows a cross section at a point downstream from FIG. 6, wherethe beam is passing between a first pair of electrodes at the samepotential, while being decelerated, and an additional pair of electrodesis provided in the middle of the gap at each end of the first pair ofelectrodes, at a voltage positive with respect to the first electrodes.In this figure, the ion beam is at a higher current, and contributessufficient space-charge to significantly perturb the electric field.

FIG. 8 shows a perspective view of a possible embodiment of thestructure of FIG. 7.

FIG. 9 shows a section similar to FIG. 7 in which a smaller high currentbeam is used, and the additional electrodes are adjusted in position tobe close to the beam.

FIG. 10 shows a partially cutaway side view of the present apparatus,with a fairly high current beam being decelerated from 10 keV to 1.5keV.

FIG. 11 shows a side view of the present invention with the potentialson the electrodes optimized to substantially compensate the space-chargeeffects shown in FIG. 4.

FIG. 12a shows a desired uniform ribbon ion beam current densityprofile, suitable for implanting with a 1-dimensional mechanical scan ofthe workpiece through the ion beam.

FIG. 12b shows a desired smooth bell-shaped spot ion beam profile,suitable for implanting with a 2D raster scan of the workpiece.

DETAILED DESCRIPTION OF THE INVENTION

Although disclosed embodiments use the terminology “deceleration” andthe apparatus is named as “deceleration apparatus”, the invention canalso be used in acceleration situations. The invention concernsapparatus which filters the neutral particles and other contaminantsaway from the ion beam while accelerating or decelerating the beam.

In this proposed approach, the ion beam is decelerated downstream fromthe mass analyzer, and downstream from a multipole magnet assemblycapable of collimating and further modifying the ion beam (such ascontrolling the beam shape, size, or uniformity). Concurrent withdeceleration, the ion beam is bent in an s-shaped path to filter out theundesired neutral particles, even the undesired charged particles withimproper charge-to-mass ratio(s).

It will be convenient to define a coordinate frame in which to describethe invention. Let the z-axis be the direction of travel of a centralreference trajectory in the ion beam. This axis may be curvilinear,following the beam path, and curvilinear axes have special propertieswhich will be mentioned when appropriate. The x- and y-axes aretransverse to the beam direction. The x-axis lies in the direction ofthe minor dimension of the ribbon beam, and the y-axis is the directionof the major dimension, as shown in FIG. 3.

FIG. 2a illustrates midplane cross section, in the z-x plane, of a priorart deceleration apparatus capable of filtering out neutral particlesdownstream of a mass analyzer. It comprises a straight-through path 501which can be utilized when deceleration is not required, and an s-shapedpath 502 utilized for deceleration. Equipotential lines in the figureillustrate how the deflection and deceleration are combined. Theelectrodes shown in section are extruded in the direction orthogonal tothe page. The beam is ribbon shaped and has a height of approximately340 mm, suitable for implanting 300 mm wafers with some overscan.

FIG. 2b illustrates a side view of the same apparatus. This view is attimes looking in the x-direction of the beam as it leaves the apparatus,and shows the equipotentials in or near the y-z surface. Because thebeam is bent, the x-direction is not a uniquely defined direction. Notethat the electrodes extend only a short way beyond the beam edges in they-direction. The electrodes are mounted in vacuum in a 2-part chambercomprising a first section 651 at a first potential (in this instance−3800V) and a second section 652 at a second potential, for convenience0V. These are linked by an insulating bushing 653. Since the electrodesextend only a finite distance beyond the beam edges, and the chamberwalls are moderately close to the electrode ends, there is somedisturbance to the potential distribution at the ends of the electrodes,and this adversely affects the direction of ion trajectories near thetop and bottom of the beam. Note that equipotential lines in the figureare curved near the top and bottom, and this demonstrates that electricfield components are present which tend to deflect the ions towards oraway from the axis, which is highly undesirable, since it disturbs theuniformity of the beam and alters its size. FIG. 4 is a cross sectionview through the beam, and while this view cannot show the defocusingcomponent of the field, it clearly shows the causes of field distortionat the electrode ends. Note the proximity of ion beam 103 to theelectrode ends.

If the beam is of sufficiently low current, the effects of space-chargeare negligible compared with the applied electric fields. Under theseconditions, the distortion of the electric field caused by the finiteelectrode length and proximity of chamber wall potentials can bemitigated by designing suitable terminating electrodes. In accordancewith this invention, FIG. 5 shows a set of electrodes terminated by capsand supplementary electrodes. The potential distribution between theelectrodes near the caps, and thus the fields, more closely approachesthat at the symmetry plane.

However, this is not sufficient to transmit uniform ribbon beams athigher currents, where space-charge forces are more significant.Poisson's equation is

${{\nabla^{2}V} = {- \frac{- \rho}{\varepsilon_{0}}}},$and in Cartesian coordinates this can be written

$\begin{matrix}{{{\frac{\partial\;}{\partial x}E_{x}} + {\frac{\partial\;}{\partial y}E_{y}} + {\frac{\partial\;}{\partial z}E_{z}}} = \frac{- \rho}{4\pi\; ɛ_{\alpha}}} & (1)\end{matrix}$From this it can be seen that a uniform current density p can give riseto linearly varying electric fields, and a possible solution would be:

$\begin{matrix}{{E_{x} = {\frac{{- \rho}\; x}{4\pi\; ɛ_{\alpha}} + {const}}}{E_{y} = 0}{E_{z} = {const}}} & (2)\end{matrix}$

This is the most desirable solution. E_(z) represents the decelerationfield, Ex represents the space-charge defocusing force added to thedeflecting force applied by the voltages on the electrodes, and E_(y) iszero. Note that the space-charge force is partially offset in thex-direction by a geometric focusing effect arising from the curvilinearz-axis, plus a further focusing effect caused by the applied fieldmodulating the energy of off-axis ions; these two focusing forces do notappear as an electric field component. For further explanation seeBanford, The Transport of Charge Particle Beams, SPON, 1966.

Thus solutions to Poisson's equation like equation 2 exist for thissystem, in which Ey is zero throughout the beam, or nearly so, and thepresent invention provides a means to realize these. The potential atthe center of an ion beam with significant space-charge p is positivecompared with that of a low-current beam. Therefore the equipotentialswithin the beam in the chicane deceleration system illustrated in FIG.2b are shifted to the left, within the beam, by the presence ofspace-charge. The invention provides a means of similarly shifting thoseparts of the equipotentials outside the beam but adjacent to it, so asto remove or at least greatly reduce the curvature of the equipotentialsat the edge of the beam, so as to satisfy equation (2). This is achievedby moving electrodes 614 a and 614 b to a position close to the edge ofthe ion beam and modifying its potential appropriately positive. Thesolution to position and potential is not exact, and is optimizedempirically by measuring the uniformity of the ion beam at a downstreamlocation by means of a traveling Faraday cup or equivalent (not shown).FIG. 11 shows the beneficial effect on these equipotentials, while FIG.7 shows a cross section illustrating how this is accomplished.

It will be readily apparent that further small changes to the potentialon the electrodes 614 a and b will cause to outermost trajectories tochange from slightly convergent to slightly divergent. Depending on theaspect ratio of the ribbon beam, this focusing/defocusing effect may bevery local, or may extend significantly into the ion beam.

When the multipole lens 402 (FIG. 1) is used to focus the beam andgenerate a spot beam of greatly reduced height, the role of electrodes614 a and 614 b is similar. They must be moved to a new position, asshown in FIG. 9, in order to be close enough to the ion beam, and sincefocusing the beam increases the space-charge density, the requiredpotentials will be more positive.

In a preferred embodiment of the invention, illustrated in FIG. 3, alldeflecting and/or decelerating electrodes in the deceleration assemblyextend further in the y-direction than the maximum extent of the beam.The maximum height of the beam is targeted to be 340 mm, to allow acomfortable overscan margin for implanting a uniform dose into 300 mmdiameter silicon wafers. Thus a ‘beam zone’ is bounded by the pathbetween the various electrodes (as in FIG. 2), and by a y-dimensionwhich may be +/−170 mm in the case of a 340 mm ribbon beam, or asignificantly smaller (for example +/−80 mm) in the case of a spot beam.The term ‘beam zone’ is used to define the zone in which the ion beam isintended to be confined, depending on circumstances. Thus the height ofthe electrodes needs to be at least 400 mm, and preferably about 500 mmor more. Cap electrodes are attached and connected to the ends of theseelectrodes, for example cap 615 a is mounted on electrode 915 at its topend, and 615 b (not shown) is similarly connected to its bottom end.These electrodes extend toward the surface x=0 (see FIG. 4 fordefinition), closing in the gap at top and bottom. This has the effectof electrostatically screening the interior zone from externalpotentials, attenuating their influence. However it also modifies theshape of the transverse electric field close to the cap electrodes,which is undesirable. The middle electrodes 910 are positioned near thepoint of inflection of the beam path, and are at the same potential, andconsequently their cap electrodes 610 a are conveniently joined.Furthermore, the potential on electrode 610 a is selected to beapproximately midway between the potentials on electrodes 915 and 911 b.(Since 911 b is concealed in FIG. 3, see FIG. 6 for a cross sectionalview. In FIG. 3 it is below cap electrode 611 a.) It is thereforebeneficial to give 610 a and 610 b a sculpted shape extending towardschamber section 651, since this causes the electric field close to thecaps to better approximate the field near the midplane. FIG. 6 and FIG.7 are cross sections in which a slice through electrode 610 a appears,and FIG. 8 shows a preferred embodiment of this electrode. FIG. 9 is across section in which the middle of electrode 610 a is cut away, andpart of movable electrode 614 a occupies its place.

In general, the shapes of the cap electrodes are optimized using codessuch as Cobham's OPERA, which solve Laplace's and Poisson's equations in3 dimension, with the goal of rendering the equipotential lines verticaland straight within the beam zone (since this eliminates any unwantedE_(y) electric field component) and additionally keeps the strength ofE_(x) as uniform as possible inside the beam zone. Laplace's equation isused in the limit of low beam currents, Poisson's equation whenconsidering high current beams with significant space-charge. Thedetailed three-dimensional shapes determine the relative effect of theelectrode potentials in the beam zone.

Electrodes 614 a and 614 b play a similar role to 610 a and b,controlling both the strength of E_(x) near the top and bottom of thebeam zone, and modifying the shape of the equipotentials to keep them asstraight as possible.

Now consider the effect of space-charge in the beam. As discussed above,its first effect is to push the center of the equipotential surfaces tothe left in the figures, and this can be clearly seen in FIG. 10, for ahigh current beam. The equipotentials become curved, and the curvaturewill be greatest where the space-charge is highest, which generally willbe where the beam energy is lowest. If a positive potential is nowapplied to electrodes 614 a and 614 b, the effect is to shift theequipotentials near these electrodes to the left, as shown in FIG. 11,which has the effect of reducing, or in some instances reversing, thecurvature of the equipotentials. The effect on the beam can be seen: itsdivergence is reduced. In FIG. 10, a substantial fraction of the beamdiverges at angles with magnitudes between 10 and 20 degrees; In FIG.11, the divergence is reduced so that for most of the beam it is lessthan 1 degree, although a small section at the edge of the beam exceedsthis limit. FIG. 7 illustrates this in cross section through the beam.

Clearly if the beam is now changed from a ribbon beam to a spot beam ofabout ⅓ the height, this approach must be modified. Electrodes 614 a andb cannot fulfill the same role unless they are moved closer to the beam.To accomplish this, in this preferred embodiment, these electrodes aremounted on a controllable servomechanism to allow them to be movedcloser to the ion beam, as shown in FIG. 9. In other embodimentsadditional electrodes are made movable, such as 610 a and 610 b.

Although specific embodiments have been illustrated and described, itwill be appreciated by those skilled in the art that the presentinvention may be applied to a variety of situations in which chargedparticle beams are accelerated or decelerated. Thus although, forexample, this invention was conceived as an enhancement to adeceleration system operating with an s-shaped beam path, manymodifications to the beam path may be conceived, and which are intendedto fall within the scope of the invention. In particular, the apparatusmay be used in any orientation. The scope of the present invention isintended to be limited solely by the appended claims.

What is claimed is:
 1. An electrode assembly for accelerating ordecelerating an ion beam, the electrode assembly comprising: a pluralityof electrodes configured to define a first ion beam path through theelectrode assembly; a first pair of deflecting electrodes of theplurality of electrodes defining a first portion of the first ion beampath, wherein: the first pair of deflecting electrodes form an openinghaving a long dimension and a short dimension perpendicular to the longdimension; the first pair of deflecting electrodes are positioned onopposite sides of a first plane aligned with the long dimension; and thepair of deflecting electrodes are configured to deflect the ion beam bya first amount with respect to the first plane as the ion beam passesbetween the first pair of deflecting electrodes; a second pair ofdeflecting electrodes of the plurality of electrodes defining a secondportion of the first ion beam path, wherein: the second pair ofdeflecting electrodes is positioned on opposite sides of a second planethat is parallel to the long dimension; the second pair of deflectingelectrodes are configured to deflect the ion beam by a second amountwith respect to the second plane as the ion beam passes between thesecond pair of deflecting electrodes; and a first pair of auxiliaryelectrodes of the plurality of electrodes defining a third portion ofthe first ion beam path between the first pair of deflecting electrodesand the second pair of deflecting electrodes, wherein: the first pair ofauxiliary electrodes are positioned on opposite sides of a third planethat is perpendicular to the first plane and to the long dimension; andthe first pair of auxiliary electrodes are configured to at leastpartially shield the first ion beam path from surrounding electricfields.
 2. The electrode assembly of claim 1, wherein the first pair ofauxiliary electrodes are electrically isolated from the first pair ofdeflecting electrodes and the second pair of deflecting electrodes. 3.The electrode assembly of claim 1, wherein each auxiliary electrode ofthe first pair of auxiliary electrodes comprises a planar electrodeplate that is parallel to the third plane.
 4. The electrode assembly ofclaim 1, wherein at least one auxiliary electrode of the first pair ofauxiliary electrodes is configured to translate in a directionperpendicular to the third plane to increase or decrease a distancebetween the first pair of auxiliary electrodes.
 5. The electrodeassembly of claim 1, wherein the plurality of electrodes furthercomprise: a second pair of auxiliary electrodes defining a fourthportion of the first ion beam path between the first pair of deflectingelectrodes and the second pair of deflecting electrodes, wherein thesecond pair of auxiliary electrodes are positioned on opposite sides ofa fourth plane that is parallel to the long dimension.
 6. The electrodeassembly of claim 5, wherein the first pair of auxiliary electrodes areelectrically isolated from the second pair of auxiliary electrodes. 7.The electrode assembly of claim 5, wherein at least one auxiliaryelectrode of the first pair of auxiliary electrodes is configured totranslate between the second pair of auxiliary electrodes in a directionperpendicular to the third plane to increase or decrease a distancebetween the first pair of auxiliary electrodes.
 8. The electrodeassembly of claim 5, further comprising: a pair of auxiliary electrodecaps attached to opposite ends of the second pair of auxiliaryelectrodes, the pair of auxiliary electrode caps positioned on oppositesides of the third plane.
 9. The electrode assembly of claim 8, whereineach auxiliary electrode of the first pair of auxiliary electrodes is atleast partially surrounded by an auxiliary electrode cap of the pair ofauxiliary electrode caps.
 10. The electrode assembly of claim 8, whereinthe first pair of auxiliary electrodes are electrically isolated fromthe pair of auxiliary electrode caps.
 11. The electrode assembly ofclaim 8, wherein the pair of auxiliary electrode caps extend towards thefirst pair of deflecting electrodes such that a portion of the pair ofauxiliary electrode caps are positioned on opposite sides of the firstportion of the first ion beam path defined by the first pair ofdeflecting electrodes.
 12. The electrode assembly of claim 8, wherein afirst electrode of the first pair of deflecting electrodes has a firstelectric potential, a second electrode of the first pair of deflectingelectrodes has a second electric potential that is different from thefirst electric potential, and the pair of auxiliary electrode caps hasan electric potential approximately equal to a mean of the firstelectric potential and the second electric potential.
 13. The electrodeassembly of claim 1, wherein the first ion beam path has an S-shapedtrajectory.
 14. The electrode assembly of claim 1, wherein the pluralityof electrodes are configured to define a second ion beam path that isdifferent from the first ion beam path, and wherein the first pair ofauxiliary electrodes define a portion of the second ion beam path. 15.An electrode assembly for accelerating or decelerating an ion beam, theelectrode assembly comprising: a plurality of electrodes configured todefine a first ion beam path through the electrode assembly; one or moreentrance electrodes of the plurality of electrodes disposed at a firstside of the electrode assembly, the one or more entrance electrodesforming a first entrance opening, wherein the first entrance opening hasa long dimension and a short dimension perpendicular to the longdimension; one or more exit electrodes of the plurality of electrodesdisposed at a second side of the electrode assembly opposite to thefirst side, the one or more exit electrodes forming an exit opening,wherein the first ion beam path extends from the first entrance openingto the exit opening; a first pair of deflecting electrodes of theplurality of electrodes, wherein: the first pair of deflectingelectrodes define a first portion of the first ion beam path; the firstpair of deflecting electrodes are positioned on opposite sides of afirst plane parallel to the long dimension of the first entranceopening, and the first pair of deflecting electrodes are configured todeflect the ion beam by a first amount with respect to the first planeas the ion beam passes between the first pair of deflecting electrodes;and a second pair of deflecting electrodes of the plurality ofelectrodes defining a second portion of the first ion beam path,wherein: the second pair of deflecting electrodes is positioned onopposite sides of a second plane that is parallel to the long dimensionof the first entrance opening; the second pair of deflecting electrodesare configured to deflect the ion beam by a second amount with respectto the second plane as the ion beam passes between the second pair ofdeflecting electrodes; and a first pair of auxiliary electrodes of theplurality of electrodes defining a third portion of the first ion beampath between the first pair of deflecting electrodes and the second pairof deflecting electrodes, wherein: the first pair of auxiliaryelectrodes are positioned on opposite sides of a third plane that isperpendicular to the first plane and to the long dimension; and thefirst pair of auxiliary electrodes are configured to at least partiallyshield the first ion beam path from surrounding electric fields.
 16. Theelectrode assembly of claim 15, wherein at least one auxiliary electrodeof the first pair of auxiliary electrodes is configured to translate ina direction perpendicular to the third plane to increase or decrease adistance between the pair of auxiliary electrodes.
 17. The electrodeassembly of claim 15, wherein the plurality of electrodes furthercomprise: a second pair of auxiliary electrodes defining a fourthportion of the first ion beam path between the first pair of deflectingelectrodes and the exit opening, wherein the second pair of auxiliaryelectrodes are positioned on opposite sides of a fourth plane that isparallel to the long dimension of the first entrance opening, andwherein at least one auxiliary electrode of the first pair of auxiliaryelectrodes is configured to translate between the second pair ofauxiliary electrodes in a direction perpendicular to the third plane toincrease or decrease a distance between the first pair of auxiliaryelectrodes.
 18. The electrode assembly of claim 15, wherein: the one ormore entrance electrodes form a second entrance opening; the pluralityof electrodes define a second ion beam path extending from the secondentrance opening to the exit opening; and the first pair of auxiliaryelectrodes define at least a portion of the second ion beam path.
 19. Anion implantation system comprising: an ion source configured to generatean ion beam; an analyzing magnet configured to mass analyze the ionbeam; at least one multipole lens configured to focus the ion beam; andan electrode assembly for accelerating or decelerating the ion beam, theelectrode assembly comprising: a plurality of electrodes configured todefine a first ion beam path through the electrode assembly; a firstpair of deflecting electrodes of the plurality of electrodes defining afirst portion of the first ion beam path, wherein: the first pair ofdeflecting electrodes form an opening having a long dimension and ashort dimension perpendicular to the long dimension; the first pair ofdeflecting electrodes are positioned on opposite sides of a first planealigned with the long dimension; and the pair of deflecting electrodesare configured to deflect the ion beam by a first amount with respect tothe first plane as the ion beam passes between the first pair ofdeflecting electrodes; a second pair of deflecting electrodes of theplurality of electrodes defining a second portion of the first ion beampath, wherein: the second pair of deflecting electrodes is positioned onopposite sides of a second plane that is parallel to the long dimension;the second pair of deflecting electrodes are configured to deflect theion beam by a second amount with respect to the second plane as the ionbeam passes between the second pair of deflecting electrodes; and afirst pair of auxiliary electrodes of the plurality of electrodesdefining a third portion of the first ion beam path between the firstpair of deflecting electrodes and the second pair of deflectingelectrodes, wherein: the first pair of auxiliary electrodes arepositioned on opposite sides of a third plane that is perpendicular tothe first plane and to the long dimension; and the first pair ofauxiliary electrodes are configured to at least partially shield thefirst ion beam path from surrounding electric fields.